Written by Dan Sardaro

March –river birch

In a monthly series about trees, it’s no surprise water is a central topic. Across the United States, water travels over land in a vast, fractal-like network of rivers and streams. But if one were to fold the country in half, it would become apparent that the eastern half of the country contains a larger river network than the western half. The Mississippi Basin alone takes up almost half the country, with its tributaries reaching far and wide into the American heartland. 

One particular family of trees has grown to thrive off that geographical divide – the birch family (Betulaceae). While there are species of birch that grow in the west, the conditions that the east provides are second to none for a tree that can often be found right beside a creek. This month, the river birch (Betula nigra) will be our focus. 

Sometimes referred to as a water birch, the river birch thrives when its roots can find an ample source of water. While the tree can also tolerate drier soils, those that are moist, fertile, and acidic are most desired. This means that the hot, humid southeast is the perfect home, but this birch ranges from New England to northern Florida and as far west as southern Minnesota and eastern Texas. Most of the other species that call the eastern United States home have not expanded to the humid climate of the southeast. Pair that with the fact that this tree is mostly immune from pests like the birch borer, and you have the makings for an extremely hardy tree specimen. 

The Structure

The river birch is a deciduous tree that can quickly reach 60-80 feet tall and 40 feet wide. At its maturity, you’ll find that four or five trunks will branch out into a pyramid structure that expands as the canopy rises. So, while it’s always been largely useless for timber, the river birch’s shady canopy is one of the reasons why it has been a staple of residential neighborhoods and city parks. 

If you know anything about birches, typically, the bark comes to mind, and the river birch has some of the most beautiful bark of its family. The makeup and color can vary among individual plants, ranging from a silvery gray-brown to pinkish-brown when young, but dark, narrow, longitudinal lenticels are always a defining marker. 

The bark around the trunk is papery and peels off in curly sheets of gray, brown, salmon, peach, orange, and lavender (but don’t attempt to rip it off yourself; there usually are flakes at the bottom of a tree if you want to observe more closely). While the branches are usually smoother, older branches can exfoliate as well. Lastly, the leaves are typical for a birch. They are toothy and diamond-shaped, colored medium green on top and a silvery green underneath, which makes for an eye-pleasing shimmer when the wind blows through them. 

The Biology

Lenticels, or small ridges that run across the branches, are pores for the tree and help exchange gases between the atmosphere and the internal tissues. Leaf cells are responsible for most of the photosynthesis and respiration for woody plants, and the underside of the leaf functions exactly for that exchange. Still, cells that comprise trunks and branches use these lenticels for the same purpose.

As I mentioned earlier, other birch species can be found here in Pennsylvania, including white (or paper), gray, black, and yellow. As you begin to take hikes in the blossoming parks and green spaces these days, these river birches will bloom and drop seeds that attract chickadees, song sparrows, wild turkeys, pine siskins, and finches. Spring’s symphony is here at last!